The effects of sustained transmitter release on the ultrastructure of presynaptic areas in lamprey Muller axons will be examined. The effects of time and temperature on this effect will be tested. Sustained transmitter release will be produced by electrical stimulation to produce action potentials, by sustained depolarization via increased extracellular K and by drugs (e.g. veratridine, guanidine, 4-aminopyridine) which cause high-frequency spontaneous action potential activity. In other experiments the motor effects of simultaneous stimulation of two or more Muller axons in a semi-intact lamprey preparation will be assessed.